Experiment - A Greener Approach to Molar Mass Determination Using Freezing Point Depression
Objectives -
- Better understanding of the fundamentals of colligative properties and their uses.
- Interpretation of a real journal article and how to write one.
- Comparison between a journal article and student experimental instructions.
- Improvement in students understanding of the experiment and their own journal article reports.
- Identify the locations of a claim and relationship to the hypothesis.
- Evaluate an Abstract.
Results -
This Results section is provided to help you with the analysis of your data. However it should not be used in place of writing your own, original JAR.
Unlike pure solvents, solutions do not normally stay at the same freezing point as they freeze because the concentration changes as the solvent freezes. This may make the freezing curve slightly sloped. To accurately determine the freezing points of pure stearic acid and each of the solutions, "cooling curves" of temperature (y-axis) vs. time (x-axis) are plotted. Data where the temperature changes by more than 0.5oC per 30 seconds is plotted as one series and data where the temperature changes by less than 0.5oC per 30 seconds as a second series. A best fit line is then calculated for each series and the freezing point is obtained by finding the intersection of the two best fit lines, see Figure. This can be done on the LabQuest however the data will still have to be plotted in a plotting program for the JAR figures. Logger Lite is a free program that can plot your LabQuest data or a program like Excel can be used.
Six plots will be needed to get your freezing points. However, only three graphs need be present in the JAR-one for each solution. Typically the worst data plot is chosen to demonstrate to the reader that the data is valid. From these six graphs determine the freezing point for the pure stearic acid and each of the unidentified sample mixtures. The plot for pure stearic acid will be the same as the plots for the mixtures except the melting point will appear at a higher temperature for the pure stearic acid then it does for the mixtures. If the slope is near zero an average of the data over a short range can be used as the freezing point instead of finding the intersection of two lines.
Calculations -
1. Determine the freezing point for each run:
a. If the cooling curve freezing data slope is near zero, an average may be used over an appropriate range after it levels out. What range should you use?
b. If the cooling curve freezing data slope is not zero, fit the lines as shown in Figure 1.3 and show the lines on the figure in your report. The freezing point is the intersection of the two lines.
2. Average the freezing point for each solution, two runs for pure solvent, two runs for ∼1 g unknown and two runs for ∼2 g unknown yielding three freezing points. Find the change in freezing points, ΔTf, for the mixtures by comparing the average freezing point of each of the solutions to the average freezing point of pure stearic acid.
3. Use Eq. (2) to find the molality of each solution and its uncertainty. Note the manufacturer specifics the probe accuracy is ±0.2oC for any measurement, you have made two measurements, and there may be some judgment in the freezing point determination.
4. Use the molalities to determine the moles of unknown sample added in each trial. Note: for the second addition of unknown sample you are finding the total number of moles added so add the grams added both times.
5. Determine the two molecular mass values of the unknown sample and its uncertainty, one from the ∼1 g solution and one from the ∼Δ 2 g solution.
6. Determine the average molecular weight with its uncertainty and identity of your unknown sample using the list below. You will have two values for the MM. Compare your average and uncertainty with values provided.
- lauric acid (dodecanoic acid) - MM 200.32 g/mol
- myristic acid (tetradecanoic acid) - MM 228.37 g/mol
c palmitic acid (hexadecanoic acid) - MM 256.26 g/mol
7. As it is likely your reader will not know the structures for stearic, lauric, myristic, and palmitic acids (did you?), you should report them in your JAR-likely in the Introduction or it could be in the Results and Discussion.
8. Attach an appendix to your JAR with the calculations for the uncertainty determination. Does your molar mass agree with the literature value within your uncertainty?
Results - The Results presentation, written in past tense, includes raw data, results of sub-sequent data analysis, and uncertainties in the quantities obtained. Sample calculations, if required, should be presented in an appendix. It is different from the Discussion section in that the data is presented objectively, without interpretation, and aims to provide evidence to support the claim.
Tables and figures should be introduced and presented with text, along with an explanation of their contents. They should not simply appear on the assumption that the reader (or grader) knows what they are about. A table should have a title and labeled headings. Units of measurement and uncertainties associated with the measurements should be presented in some manner. Below is an example of a table for a paper chromatography experiment.
Discussion and Conclusion (usually present tense though it can be past). In some situations, particularly advanced courses or articles, the Discussion is included with the Results and the Conclusion is separate. This sometimes makes the flow of discussing the results easier. However, we are explicitly keeping the Results as a separate section to clarify the objective nature of the results. Interpretations and evaluations of the results are presented together in this Discussion and Conclusion section. Generally, claims (conclusions) are in the last paragraph of this section. This section is usually presented in present tense, although past tense is also acceptable.
Consider this section as the final stage in building a scientific argument. Here is where the whole report comes together. The Discussion links together the question raised in the experiment, the approach to address the question, the evidence gathered from experiments, and the implications and interpretations leading to the reasoning for the final claim the answer to the question. Here are some points to consider (but not as a list!):
What question were you trying to answer and why?
What did you do during your investigation and why did you conduct your investigation this way?
Discuss the data and as you do, refer to the tables and figures presented in the Results section. Discuss any calculations made and the quantities derived from calculations.
What implications and interpretations can be derived from the results? What is your argument or claim? Use reliable references to support your claim. Do your results agree with expectations arising from a theory or from earlier work by other scientists?
Compare your numerical results with literature values. Numerical data previously reported in the scientific literature often serves as an accepted or literature value with which your work may be compared.
Discuss possible sources of uncertainty that may affect your reported experimental value(s) including the direction and magnitude of each effect and suggest ways to minimize or eliminate these uncertainties. "Human error" is not an acceptable explanation for disagreement.
The last one or two paragraphs of this section must be your conclusion. This is where the final claim or claims of the paper are clearly stated. Conclusion, answer, or claim are all synonyms for the answer to the problem originally posed.
The claim must match the original question asked-or change one or the other.
Although the claim must match the original question, it may not agree with the pre-diction. For example, the hypothesis may predict that ozone levels are increasing with time but the data indicates they are actually decreasing. They match in question but the answer doesn't agree with the prediction.
The claim must match the data and results-or change the claim or indicate why the data is incorrect.
Attachment:- Experiment.rar
Attachment:- Assignment Files.rar